1. Field of the Invention
This invention relates to an improved plastic packaging or wrapping film, and more particularly, to implosion-resistant shrink, skin, stretch and vacuum wrap films with improved clarity, toughness, extrusion processibility and irradiation cross-link efficiency. The films contain substantially linear ethylene polymers and may be biaxially oriented, multilayered and/or constructed with barrier characteristics.
2. Technical Background
Food items such as poultry, vegetables, fresh red meat, and cheese, as well as nonfood industrial and retail goods, are packaged by shrink, skin, stretch and/or vacuum wrap methods. The shrink packaging method involves placing an article(s) into a bag fabricated from heat-shrinkable film material, then closing or heat sealing the bag, and thereafter exposing the bag to sufficient heat to cause shrinking of the bag and intimate contact between the bag and article. The heat can be provided by conventional heat sources, such as heated air, infrared radiation, hot water, combustion flames, or the like. Shrink wrapping of food articles helps preserve freshness, is attractive, hygienic, and allows closer inspection of the quality of the packaged food. Shrink wrapping of industrial and retail goods, which is alternatively referred to in the art and herein as industrial and retail bundling, preserves product cleanliness and also is a convenient means of bundling for accounting purposes.
The skin packaging method involves placing the product to be packaged on porous or perforated paperboard which is typically coated with an adhesive primer, then moving the loaded board to the platen of a skin packaging machine where a skin packaging film is heated until it softens and droops, relaxes and droops a second time over the loaded board. A vacuum then draws the film down around the product to provide a "skin" tight package. Skin packaging serves both the consumer retail and the transit markets. In the transit market, skin packaging protects industrial goods during transport and distribution. In the retail market, skin packaging protects consumer goods against damage and pilferage as well as provides "display appeal" to maximize the sales potential of the packaged product. While most, if not all, nonfood skin packaging film is monolayer, multilayer skin packaging films are useful for protecting food by vacuum packaging and, especially by vacuum skin packaging.
Food items are also packaged by the stretch wrapping method which involves manually pulling a film over a paper pulp or foamed polystyrene tray filled with food (or automatically pushing the tray upward to stretch the film) arid then heat sealing the stretched film at its edges usually on the underside of the tray, and allowing the film to remain taut due to its elasticity. For nonfood stretch wrapping, the stretch wrap film is manually or automatically pulled and stretched over and/or around the product, and thereafter the free end of the film is clung or tacked (rather than heat sealed) to another portion of film already wrapped about the product or to the product itself usually by applying pressure in the direction towards the product or goods being wrapped. Stretch wrap packaging of fresh food is specific to the consumer retail market and it allows fresh red meat to bloom to the desired bright red color as well as allows some vegetables to appropriately respire. Stretch wrapping of nonfood items corresponds to the transit market, and includes pallet wrapping of goods as well as wrapping of new vehicles during distribution to protect exterior paint finishes from damage due to acid rain, road chips, debris, vandalism, etc.
Whereas stretch wrap packaging typically does not involve barrier film layers and is useful for both food and nonfood items, vacuum packaging involves a gas or oxygen barrier film layer and is generally reserved for red meats, processed meats and cheeses, but is also used to package odor-sensitive or odor-generating nonfood items such as cedar wood chips. There are several methods or variations of vacuum packaging including vacuum skin packaging which is also referred to in the art as vacuum form packaging. One method involves, for example, bringing a heat-softened top and bottom film web together under vacuum in a chamber with the product loaded between the webs; thereafter, heat sealing the webs together at their edges, and then evacuating or gas flushing the space containing the product. In vacuum packaging, typically the bottom web takes up the form of the food item being packaged.
While the shrink wrapping method is predicated on the heat-shrinking properties of the selected film materials, stretch overwrapping is predicated on the elasticity of the film material. Conversely, successful skin packaging is predicated on the adhesion of the film material to the primed board and the amount of time required to cause the film to double droop (cycle time). Similar to skin packaging, successful vacuum packaging depends on the time required for the film webs to sufficiently soften before being drawn by vacuum (or pushed by air pressure) about the product to be packaged. As taught in Plastics Design and Processing, November 1980, page 4, film materials with more infra-red heat absorption bands and/or with a lower Vicat softening point will tend to heat-up and soften faster, and thereby allow faster cycle times in skin and vacuum packaging. In general, polar polymers such as, for example, ethylene vinyl acetate (EVA) copolymers, ethylene acrylic acid (EAA) copolymers and ionomers, will possess more infra-red heat bands than nonpolar polymers such as the substantially linear ethylene polymers of the present invention or heterogeneous LLDPE. Further, ionomers show more infra-red heat bands than their respective base copolymers due the ionomerization itself.
Successful packaging or wrapping for all four methods, depends on the toughness and abuse or implosion resistance properties of the film materials themselves such that the packaged product's integrity is maintained during distribution, handling and/or display. However, toughness and abuse resistance are particularly important in food shrink wrapping and vacuum packaging which often times involves packaging of meat and other food cuts with deep cavities and sharp exposed bones as well as exposed edges that can puncture the film webs or fabricated bag during the heat-shrink or vacuuming-form operation or during subsequent package handling and distribution. To avoid premature puncturing, film producers resort to expensive practices to toughen the package such as using thicker films and bags, using an extra layer of film at critical contact points of the bag in a patch-like fashion as described by Ferguson in U.S. Pat. No. 4,755,403, or by using cross-ply or non-parallel layer constructions. Similarly, to "artificially" enhance the puncture and other abuse or implosion resistance characteristics of known film materials, food packagers routinely wrap or cap exposed bone edges with cloth, molded plastic articles or other materials.
An important shrink bundling and skin packaging property, particularly for delicate items or items which tend to crush or bend, such as paper goods, is the tension or force the film exerts on the packaged article and/or board. This attribute is known in the art as shrink tension, and films with too much shrink tension invariably yield shrink or skin packages with unsightly buckling or board curl that in severe cases can render the packaged good unusable for its intended purpose. In addition to being aesthetically unsightly, buckled or warped goods are difficult to stack uniformly on display shelves.
The film optical properties are also important for retail "point-of-purchase" shrink, skin, stretch and vacuum wrap packages. The better the contact and/or see-through clarity, the lower internal film haze and the higher film gloss or sparkleness, the more likely the package will attract a potential purchaser for closer inspection. Further, consumers generally associate the package aesthetics, which are chiefly predicated on the optical properties of the packaging film, directly with the quality of the article to be purchased.
Another important retail "point-of-purchase" requirement, that is specific to stretch wrapping, is the ability of the film to "snap back" when deformed rather than retain the dents and impressions left from inspections by prospective purchasers. This attribute is predicated on the elastic recovery of the film material, and when elastic recovery is sufficiently high, subsequent prospective purchasers are not unnecessarily prejudiced by the package appearing as if it had been handled and repeatedly rejected.
Still another important film material characteristic, that can affect the overall success of all four packaging and wrapping methods, is the extrusion processibility of the film resin during film fabrication by well known bubble, cast or sheet extrusion methods. Good processibility is manifested as relatively low extrusion energy consumption, a smoother film surface and as a stable bubble or web even at higher blow-up ratios, draw rates and/or film thicknesses. There are numerous benefits of a smoother, more stable film-making operation, including film widths and thicknesses are generally more uniform, the need to edge trim is reduced (which reduces waste), winding and unwinding operations are typically smoother, there are fewer film wrinkles, and the final package quality or appearance is improved.
While high pressure polymerized ethylene homopolymers and copolymers, such as low density polyethylene (LDPE) and ethylene vinyl acetate (EVA) copolymers, generally exhibit good processibility during extrusion as the consequence of having relatively high degrees of long chain branching, linear olefin polymers such as linear low density polyethylene (LLDPE) and ultra low density polyethylene (ULDPE), which is alternatively known in the art as very low density polyethylene (VLDPE), show fair-to-marginal processibility even when fairly sophisticated extrusion screw designs such as barrier screws, screws with Maddock mixing sections, and other like variations are employed to better homogenize or stabilize the polymer melt stream and allow lower energy consumption and smoother polymer surfaces. Further, in attempts to maximize the toughness characteristics of known EVA, ULDPE and LLDPE materials, it is common practice to employ very high molecular weight grades, e.g. melt indices (I.sub.2, as measured in accordance with ASTM D-1238 (190.degree. C./2.16 kg)) of .ltoreq.0.5 g/10 minutes, which inevitably adds to processibility difficulties.
To meet the diverse performance requirements involved in all four packaging and wrapping methods, various film materials have been used as single components and in blended combinations for both monolayer and multilayer packaging. For example, Smith in U.S. Pat. No. 5,032,463 discloses biaxially stretched monolayer and multilayer films comprising blends of ethylene/1-butene ultra low density polyethylene and ethylene/1-hexene ultra low density polyethylene.
As another example, Lustig et al. in U.S. Pat. No. 5,059,481 describe biaxially oriented ultra low density polyethylene monolayer and multilayer packaging films with a barrier core layer, an ethylene/vinyl acetate intermediate layer and ULDPE/EVA blends as the outer layer. In U.S. Pat. No. 4,863,769, Lustig et al. disclose the use these biaxially oriented ultra low density films as bags for packaging frozen poultry, and in U.S. Pat. No. 4,976,898, Lustig et al. disclose that the "double bubble" method can be used to prepare the biaxially oriented ultra low density polyethylene films.
In another example, Botto et al. in European Patent Application 0 243 510 and U.S. Pat. No. 4,963,427 describes a multilayer skin packaging film consisting of an ionomer, EVA and HDPE that is particularly useful for vacuum skin packaging of food.
While prior art film materials have varying degrees of toughness, implosion resistance, low temperature shrinking characteristics, and bag making heat sealing performances, even tougher film materials are desired in shrink, skin and vacuum packaging for reduced bag punctures or for maintaining puncture resistance levels when down-gauging film thicknesses for environmental source reduction purposes, cost-effectiveness or other considerations. Moreover, while low density polyethylene (LDPE) produced via free radical, high pressure polymerization of ethylene performs satisfactorily in industrial (transit) shrink and skin packaging applications, the optical properties of LDPE generally are not satisfactory for consumer retail packaging applications and in the instance of retail skin packaging, packagers are left to rely on expensive film materials, such as Surlyn.TM. ionomers supplied by E. I dupont, for the desired optical appeal. However, even the expensive ionomer products show skin packaging deficiencies such as poor biaxial tear/cut resistance and insufficient drawability that can yield aesthetically unpleasing ridges and/or bridges when multiple items are packaged on a single paperboard.
Although having poor tear/cut resistance in both the machine and transverse directions is clearly an ionomer disadvantage, there is benefit to reduced tear/cut resistance in one direction or another, i.e., to facilitate easy opening of the package while maintaining its tamper-evident quality.
The search for an alternative to polyvinyl chloride (PVC) films for stretch wrap for food is another example of packagers having to rely on expensive film materials. Such alternatives have typically been olefin multilayer film. The search is impartant, however, because PVC has undesirable plasticizer migration tendencies as well as a growing environmental concern regarding chlorinated polymers in general. While various multilayer films have been disclosed (for example, in U.S. Pat. Nos. 5,112,674 and 5,006,398, and in EPO 0 243 965, EPO 0 333 508, and EPO 0 404 969) with similar snap-back or elastic recovery as PVC, many of these solutions involve coextrusions with ethylene copolymers such as ethylene vinyl acetate (EVA) and ethylene acrylic acid (EAA) copolymers. Use of these polar copolymers presents processing limitations including thermal stability and recycle/trim incompatibility.
Another desired improvement over known olefin polymers is disclosed in EPO 0 404 368 where Ziegler catalyzed ethylene .alpha.-olefin copolymers, such as ethylene/1-butene, ethylene/1-hexene, and ethylene/1-octene copolymers are shown to require blending with LDPE to provide film materials with adequate shrink properties (especially in the cross direction) when processed via simple blown film extrusion.
In providing film materials with improved toughness and abuse or implosion resistance characteristics for shrink packaging, good low temperature heat-shrink performance in both the machine and cross directions must also be provided. Also, for shrink and skin packages void of excessive curl or warpage, shrink tension must be maintained at a low level, and to achieve the desired free shrink characteristics, the film material must possess the morphology and be strong enough to withstand the physical biaxial stretching that occurs during film fabrication in the simple bubble extrusion process or in more elaborate processes such as the double bubble process described by Pahlke in U.S. Pat. No. 3,555,604, the disclosure of which is incorporated herein by reference. Improved film materials must also exhibit good processibility and optical properties relative to known film materials, and particularly, relative to the very low density polyethylene (VLDPE) materials and films disclosed by Lustig et al. in U.S. Pat. Nos. 5,059,481; 4,863,769; and 4,976,898.
Mitsui Petrochemical has been selling products prepared by polymerizing ethylene and a higher .alpha.-olefin under the trademark "Tafmer.TM." for more than a decade that are considered to be a class of very low modulus VLDPE materials. Some of the Tafmer.TM. grades have been marketed for use in multilayer film packaging structures. For example, U.S. Pat. No. 4,429,079 (Shibata et al.) assigned to Mitsui Petrochemical Industries, the disclosure of which is incorporated herein by reference, discloses a composition in which a random ethylene copolymer (conventional LLDPE having one, two or more melting points from 115.degree. C. to 130.degree. C. labeled as component (A) is blended with another random ethylene copolymer (one having a single melting point from 40.degree. C. to 100.degree. C.), labeled as component (B) to provide compositions where component (B) does not exceed 60 percent by weight of the total composition with improved properties, in particular, improved low-temperature heat sealability and flexural toughness for resisting pinhole formation during handling. However, with excellent heat sealability and flexibility notwithstanding, Tafmer.TM. products are not generally recognized or marketed as having excellent abuse resistance properties and shrink characteristics. The Tafmer.TM. products having a single melting point are homogeneously branched linear polyethylenes which were earlier described by Elston in U.S. Pat. No. 3,645,992 and are made by a related polymerization process using vanadium catalysts.
Exxon Chemical Company has recently introduced products similar to Mitsui Petrochemical's Tafmer.TM. products which Exxon prepared by polymerizing ethylene and an .alpha.-olefin (e.g., 1-butene n)-hexene) in the presence of a single site metallocene catalyst. In a paper presented on Sep. 22-27, 1991 at the 1991 IEEE Power Engineering Society Transmission and Distribution Conference ("New Specialty Linear Polymers (SLP) For Power Cables", printed in the proceedings on pp. 184-190) in Dallas, Tex., Monica Hendewerk and Lawrence Spenadel, of Exxon Chemical Company, reported that Exxon's Exact.TM. polyolefins polymers, said to be produced using single site metallocene catalyst technology, are useful in wire and cable coating applications. Also, in the 1991 Polymers, Laminations & Coatings Conference Proceedings, pp. 289-296 ("A New Family of Linear Ethylene Polymers Provides Enhanced Sealing Performance" by Dirk G. F. Van der Sanden and Richard W. Halle, (also published in February 1992 TAPPI Journal)), and in ANTEC '92 Proceedings, pp. 154-158 ("Exact.TM. Linear Ethylene Polymers for Enhanced Sealing Performance" by D. Van der Sanden and R. W. Halle), Exxon Chemical describe their new narrow molecular weight distribution polymers made using a single site metallocene catalyst as "linear backbone resins containing no functional or long chain branches." Films made from the polymers produced by Exxon are also said to have advantages in sealing characteristics as measured by hot-tack and heat-seal curves, but these publications do not discuss shrink characteristics. The new Exxon polymers are said to be linear and to have narrow molecular weight distributions, and, because of the narrow molecular weight distribution, are also said to have "the potential for melt fracture." Exxon Chemical acknowledged that "it is well known that narrow-MWD polymers are somewhat more difficult to process".
Accordingly, although new materials have been recently developed and marketed for flexible packaging or wrapping purposes, the need still exists for improved olefin packaging films and bags or wraps fabricated therefrom, with particular improvement needed in regard to recovery, shrink characteristics, vacuum drawability abuse or implosion resistance and processibility relative to the VLDPE olefin polymers with linear backbones such as those described by Lustig et al. in U.S. Pat. Nos. 4,863,769; 4,976,898 and 5,059,481.